This invention relates to an improved method for the manufacture of insulated containers, particularly water heaters.
Electric and gas water heaters are generally comprised of a glass-lined, metal tank usually in the form of a cylinder which is surrounded by an insulating material retained about the tank by means of a protective, outer, sheet metal shell. In gas models, a burner compartment is normally located below the tank within the protective sheet metal shell. In electric models, a heating element normally projects through a side wall of the tank.
In order to improve the efficiency of such water heaters, it is desirable to provide external insulation for the tanks between the tank and the outer shell. Heretofore Fiberglas insulation has proven to be an excellent insulating material. More recently, expanded urethane foam has been utilized as the insulating material surrounding a water heater tank.
One of the problems associated with such a water heater assembly, particularly one having urethane foam as the insulating material, has been the method by which the foam material is formed about the tank. Generally the foam is injected as a liquid into the annular space between the inner tank and outer shell. However, it is necessary to use a core piece or other mold forming device within the space between the inner tank and outer protective shell to retain the foam within a desired region. For example, in an electric model the heating rod and controls are attached to the side wall of the tank and cannot be covered with foam.
One way of preventing foam from covering or forming over the controls has been to pack the region around the controls with Fiberglas insulation material. The Fiberglas insulation material then serves as a barrier during the foam molding operation.
Another procedure has been adopted by State Industries, Inc. of Ashland City, Tenn. The State Industries procedure calls for forming a flexible plastic bag with an open top, closed sides and a closed bottom edge. The bag then attaches to and partially encircles the inner tank. Expandable urethane foam is next injected into the bag causing the bag to expand and fill the space between the inner tank and the outer shell of the heater. In practice, the bag extends only partially around the circumference of the tank thus leaving a space or void in the region between the inner tank and outer shell defined by the opposite ends of the bag. Controls, inlets, outlets and the like are positioned in the void region, and the spaces between the controls are appropriately packed with a Fiberglas insulation material.
Another procedure is described in U.S. Pat. No. 4,372,028 issued Feb. 8, 1983 for a Method of Manufacturing Foam Insulated Tank. In that patent, the use of a small flexible bag to define the control space and the boundary for flow of foam material is disclosed. That invention permits utilization of expandable foam over substantially the entire outside surface area of a water heater tank. The method insures accurate and easily reproducible positioning of voids or spaces in the insulation through which controls, inlets, outlets and the like may be affixed to the tank. The method also eliminates usage of a large plastic bag from the manufacturing process and substitutes usage of a smaller, less expensive bag. As a result, undesired folds which may form in the large bag are avoided. Also, the insulating capability of the foam is enhanced since the foam will bond directly to the walls of the container and will not be retained in an air permeable bag. U.S. Pat. No. 4,372,028 is incorporated herewith by reference.
The present invention is an improvement of the subject matter of U.S. Pat. No. 4,372,028 and is especially useful in the manufacture of electric water heaters. The present invention eliminates the need for a secondary, separate injection of foam material as described in U.S. Pat. No. 4,372,028. Additionally, there is less leakage of foam material as well as less possibility for distortion of the flexible bag during the manufacturing process.
Thus, it is an object of the invention to provide an improved method of manufacturing a multiple layer container of the type having an inner wall, an intermediate insulating layer and an outer protective wall.
A further object of the present invention is to provide a method of manufacturing a water heater tank having an inner metal tank, an intermediate layer of expanded urethane foam material and an outer protective shell or cover.
Still another object of the present invention is to provide a method of manufacturing a multiple layer container having improved insulating features, improved economy of manufacture, and which does not require significant capital investment in new equipment.
Another object of the invention is to eliminate the necessity of using a large, tank encompassing bag to receive foam and surround an inner metal tank in an assembly which incorporates an inner tank, an insulating layer on the tank and an outer shell over the insulation layer.
A further object of the invention is to provide a method of manufacturing a foam insulated tank assembly having a single foam injection step.
These and other objects advantages and features of the invention will be set forth in the detailed description which follows.